1. Field of the Invention
The present invention relates to an automatic fine tuning control in a television receiver. More specifically, the present invention relates to an improved automatic fine tuning circuit suited for implementation in an integrated circuit in a television receiver employing a synchronous detector as a video detector.
2. Description of the Prior Art
Recently a synchronous detector has been widely used as a video detector in television receivers. The reason is that a synchronous detector has the advantages of high detection efficiency and less generation of undesired harmonics and beats. In addition, a synchronous detector operates with a low input and is easy of coupling to a low gain video intermediate frequency amplifier. For this reason, a synchronous detector has been much more employed particularly in color television receivers implemented by integrated circuits.
FIG. 1 shows a major portion of a conventional color television receiver implemented by integrated circuits and employing a synchronous detector as a video detector. Referring to FIG. 1, a television broadcast signal as received by an antenna is applied to a tuner 1. As well known, the tuner 1 typically comprises a high frequency amplifier for selectively amplifying a selected high frequency signal of a carrier frequency corresponding to the respective channel, a local oscillator for generating a local oscillation signal of a frequency different by a predetermined frequency difference from the carrier frequency of the selected high frequency signal, and a mixer for mixing the selected high frequency signal with the local oscillation signal for providing an intermediate frequency signal of the frequency commensurate with the said difference frequency. The intermediate frequency signal is obtained from the mixer of the tuner. The local oscillator comprises a tuning circuit comprising a voltage controlled variable reactance device such as a voltage controlled variable capacitance diode. As to be described subsequently, the voltage controlled variable reactance device of the local oscillator is supplied with an automatic fine tuning control signal to achieve an automatic fine tuning operation. The tuner 1 is also structured to be responsive to an automatic gain control signal to be controlled of the gain thereof, as well known to those skilled in the art. The intermediate frequency signal as obtained from the mixer of the tuner 1 is applied to a first video intermediate frequency amplifier 2. The first video intermediate frequency amplifier 2 is also structured to be responsive to an automatic gain control signal to be controlled of the gain thereof. The intermediate frequency signal as amplified by the first video intermediate frequency amplifier 2 is applied through an interstage coil 3 to a second video intermediate frequency amplifier 4. As well known, the intermediate frequency signal includes a video intermediate frequency signal of a video carrier signal of a predetermined video carrier frequency as amplitude modulated and a sound intermediate frequency signal of a sound carrier signal of a predetermined frequency lower than the said video carrier frequency by a predetermined frequency difference commensurate with a sound intercarrier frequency. The former intermediate frequency signal is referred to as "video intermediate frequency signal" and the latter intermediate frequency signal is referred to as "sound intermediate frequency signal" in the present specification. Similarly, the former carrier signal is referred to as "video carrier signal" and the latter carrier signal is referred to as "sound carrier signal" in the present specification.
The intermediate frequency signal as amplified by the second video intermediate frequency amplifier 4 is applied to a synchronous detector 5. As well known, a synchronous detector serves to multiply a video carrier signal with a video intermediate frequency signal to provide a detected video signal by way of a product. To that end, a synchronous detector comprises means for generating a video carrier signal of the same phase and frequency as that of the video intermediate frequency signal and means for multiplying the video carrier signal with the video intermediate frequency signal. The said carrier signal generating means may comprise a local oscillator responsive to the video intermediate frequency signal to be operable in synchronism with the video intermediate frequency signal. Alternatively, the video carrier signal generating means may comprise a video carrier tuned circuit of a high quality factor coupled to the video intermediate frequency amplifier. Referring to FIG. 2, a typical synchronous detector employing a video carrier tuned circuit of a high quality factor is shown. Referring to FIG. 2, the video intermediate frequency signal as obtained from the second video intermediate frequency amplifier 4 is applied to a video carrier tuning circuit 5a and the output from the video carrier tuning circuit 5a and the said video intermediate frequency signal per se are applied to a multiplier 5b. The multiplier 5b serves to multiply the video carrier signal by the video intermediate frequency signal to provide a detected video signal by way of a product, as well known to those skilled in the art.
The detected video signal is applied to a video amplifier 6 and the output therefrom is withdrawn to be utilized to drive a video circuit, as well known. The video signal as obtained from the video amplifier 6 is further applied to an automatic gain control detector 7, wherein a signal representative of the magnitude of the video signal is obtained by way of an automatic gain control signal. The automatic gain control signal thus obtained is applied to an automatic gain control amplifier 8 and the automatic gain control signal as amplified by the amplifier 8 is applied to the video intermediate frequency amplifier 2 as an automatic gain control signal. The automatic gain control signal as obtained from the amplifier 8 is further applied through an automatic gain control delay circuit 9 to the tuner 1 as a delayed automatic gain control signal, as well known.
It is pointed out that in FIG. 1 television receiver the blocks 2, 7, 8 and 9 are implemented in an integrated circuit as encircled in a dotted line as 12 and the blocks 4, 5, 6 and 10 are also implemented in a separate integrated circuit as encircled in a dotted line as 13.
Referring again to FIG. 2, the output from the video carrier tuning circuit 5a is applied to a video carrier amplifier 10 to amplify the video carrier signal to the magnitude sufficient enough to drive a so called automatic fine tuning control circuit 11. Typically, the automatic fine tuning control circuit 11 comprises a frequency discriminator adapted to discriminate the frequency shift of the video carrier signal from the predetermined video carrier frequency. As well known, the frequency discriminator 11 serves to detect a drift of the video carrier frequency of the video intermediate frequency signal to provide a correction signal, which is applied to the voltage controlled variable reactance device of the local oscillator in the tuner 1, thereby to collect the drift of the video carrier frequency.
According to the conventional automatic fine tuning circuit described with reference to FIGS. 1 and 2, a video carrier signal is obtained from a video carrier tuning circuit 5a. In fact, this makes it possible to obtain with certainty the video carrier signal in a relatively large level on the occasion of normal operation by the receiver, thereby to ensure a proper automatic fine tuning operation. Nevertheless, a disadvantage is encountered that if and when the local oscillation frequency of the tuner is shifted from a normal frequency on the occasion of a transient operation such as at the time of channel switching, at the time of turning on of a power supply, and the like, a pseudo carrier signal is generated through intermodulation of the video and sound carrier signals, which pseudo carrier signal causes malfunction in an automatic fine tuning circuit 11. Therefore, the cause of occurrence of such a pseudo carrier signal in the conventional television receivers as shown in FIGS. 1 and 2 is first considered, and then the malfunction of the automatic fine tuning control circuit because of such a pseudo carrier signal will be considered.
In general, the local oscillation frequency of a local oscillator in the tuner 1 has a tendency of being shifted away from a normal frequency corresponding to the respective channel on the occasion of the above described transient operation such as at the time of channel switching, at the time of turning on of a power supply, and the like. If and when the local oscillation frequency of the local oscillator in the tuner is shifted in the higher direction on such occasion, then the sound carrier signal could cause malfunction of the automatic fine tuning circuit, as well known. On the other hand, if and when the local oscillation frequency of the local oscillator is shifted in a lower direction, then the above described pseudo carrier signal could cause malfunction of the automatic fine tuning circuit, as well known. As to be more fully described subsequently, such pseudo carrier signal is caused by virtue of internal modulation or mutual modulation of the video carrier signal and the sound carrier signal in the video intermediate frequency amplifiers 2 and 4, as well known to those skilled in the art.
FIG. 3 shows an overall frequency characteristic (A) of the video intermediate frequency circuit including the first video intermediate frequency amplifier 2, the interstage coil 3 and the second video intermediate frequency amplifier 4, and a frequency characteristic (B) of the video carrier tuning circuit 5a in the synchronous detector 5. FIG. 3 shows the characteristic of an example of the Japanese television standard, in which the video carrier frequency is 58.75MHz, and the sound carrier frequency is 54.25MHz, the difference being 4.5MHz. Referring to the overall frequency characteristic (A) of the video intermediate frequency circuit, the circuit is adjusted such that the video carrier frequency f.sub.p comes to the intermediate of the right side slope, an absorbing bottom point of an adjacent channel trap comes to the bottom of the right side slope, and an absorbing point of the sound trap comes to the bottom of the left side sope. The pseudo carrier signal of the frequency higher by the video/sound frequency difference appears at the frequency point spaced apart from the video carrier frequency in the higher frequency derection.
With reference to FIG. 3, now consider a case where a local oscillation frequency f.sub.L becomes lower by approximately 4.5MHz of the said video/sound frequency difference. Then, it follows that the video carrier signal f.sub.p moves from the frequency points p1 and p2 on the characteristic curves (A) and (B) to the frequency points p1' and p2' on the characteristic curves (A) and (B), respectively. Accordingly, the frequency of the video carrier signal f.sub.p approaches an absorbing frequency point of the sound trap in the frequency characteristic curve (A) in the video intermediate frequency circuit, whereby the gain of the shifted video carrier signal f.sub.p by the video intermediate frequency is decreased. On the other hand, it is recalled that the quality factor of the video carrier frequency tuning circuit 5a is very high and steep, as seen by the frequency characteristic curve (B). Because of a decreased gain of the video intermediate frequency circuit at the absorbing frequency point by the sound trap and the high quality factor tuning frequency characteristic of the video carrier frequency tuning circuit 5a, the shifted video carrier signal f.sub.p obtained from the carrier frequency tuning circuit 5abecomes extremely small, if and when the local oscillation frequency f.sub.L becomes lower by approximately the said difference frequency of 4.5MHz. As a result, the detection efficiency of the synchronous detector 5 becomes very low. Nevertheless, a high quality factor of the tuning frequency characteristic curve (B) of the video carrier frequency tuning circuit 5a is necessarily required from the standpoint of a requied high detection efficiency of the synchronous detector 5 on the occasion of a normal operation of a television receiver.
If and when the detection efficiency of the synchronous detector 5 becomes lower, than the input to the video amplifier 6 becomes small, and the output from the automatic gain control detector 7 also becomes small. As a result, the tuner 1 and the first video intermediate frequency amplifier 2 that are controlled with the outputs from the automatic gain control delay circuit 9 and the automatic gain control amplifier 8, respectively, operate with the maximum gain.
As briefly described previously, since the intermediate frequency signal obtained from the tuner 1 includes the video carrier signal f.sub.p and the sound carrier signal f.sub.S the frequency of which is lower than that of the video carrier signal by the frequency difference of 4.5MHz, both carrier signals cause internal modulation or mutual modulation in the video intermediate frequency circuit, as well known to those skilled in the art, thereby to generate a pseudo carrier signal f.sub.R the frequency of which is higher than that of the video carrier signal f.sub.p by the frequency difference of 4.5MHz. The above described internal modulation causing such pseudo carrier signal f.sub.R is conspicuous, particularly in case where the video intermediate frequency circuit is implemented with an integrated circuit. In the example of the FIG. 1 diagram, two portions 12 and 13 encircled with dotted lines are each implemented with a single chip integrated circuit as described previously. In addition, the above described first video intermediate frequency amplifier 2 operates with the maximum gain, if and when the local oscillation frequency f.sub.L of the tuner 1 becomes lower than the normal frequency corresponding to the respective channel by the said frequency difference of 4.5MHz. It should be noted that just at that time the pseudo carrier signal f.sub.R has been shifted from the frequency points s1 and s2 to the frequency points s1' and s2' along the frequency characteristic curves (A) and (B), respectively, in FIG. 3. As is clear from FIG. 3, it would be appreciated that even if the pseudo carrier signal f.sub.R per se generated in the video intermediate frequency circuit is smaller than the shifted video carrier signal f.sub.p, that has now come to the absorbing frequency point of the sound trap, the shifted pseudo carrier signal f.sub.R obtained from the video carrier frequency tuning circuit 5a is larger than the shifted video carrier signal f.sub.p obtained from the video carrier frequency tuning circuit 5a.
As the pseudo carrier signal f.sub.R obtained from the video carrier frequency tuning circuit 5a becomes larger than the video carrier signal f.sub.p obtained from the video carrier frequency tuning circuit 5a, it follows that the pseudo carrier signal f.sub.R comes to be frequency discriminated by the automatic fine tuning control circuit 11 and an output voltage by virtue of the pseudo carrier signal f.sub.R comes to appear from the automatic fine tuning control circuit 11.
FIG. 4 shows a frequency versus output voltage characteristic of the automatic fine tuning circuit 11. Referring to FIG. 4, it is seen that the normal S letter shaped characteristic curve S.sub.P by virtue of the video carrier signal f.sub.p appears centering on the prescribed video carrier frequency of 58.75MHz and in addition another similar S letter shaped characteristic curve S.sub.R by virtue of the said pseudo carrier signal f.sub.R has appeared centering on the frequency lower than the said video carrier frequency by the frequency difference of 4.5MHz. In actuality, still another S letter shaped characteristic curve s.sub.S by virtue of the sound carrier signal f.sub.S appears centering on the frequency higher than the video carrier frequency by the frequency difference of 4.5MHs, in case where the local oscillation signal f.sub.L is shifted from the normal frequency corresponding to the respective channel toward a higher frequency. Referring to FIG. 4, there is shown as a curve (A) the characteristic of the voltage applied to the voltage controlled variable reactance device in the local oscillator in the tuner 11 versus the local oscillation frequency of the local oscillator of the tuner 11 in case where the local oscillation frequency is shifted in the lower frequency direction. Thus, it would be appreciated that if and when the local oscillation frequency f.sub.L is shifted toward a lower frequency it could happen that the automatic frequency tuning control becomes stabilized at the frequency point Q, resulting in occurrence of malfunction in the automatic fine tuning control circuit 11. It should be noted that the present invention is aimed to eliminate such malfunction in the automatic fine tuning control circuit 11 that could be caused by virtue of the pseudo carrier signal f.sub.R.
From the foregoing description, it has become apparent, that in case of a television receiver employing a synchronous detector as a video detector, if and when the local oscillation frequency of the tuner is shifted from the normal frequency corresponding to the respective channel toward the lower frequency on the occasion of a transient operation such as at the time of channel switching, at the time of turning on of the power supply, and the like, a pseudo carrier signal f.sub.R becomes dominant as compared with the video carrier signal f.sub.p at the output of the video carrier frequency tuning circuit 5a in the synchronous detector because of a required high quality factor characteristic of the video carrier frequency tuning circuit, which pseudo carrier signal f.sub.R could cause malfunction in the automatic fine tuning circuit 11.
In order to eliminate such malfunction in the automatic fine tuning circuit, the following two approaches could be considered. The first approach is to disable temporarily the automatic fine tuning control circuit on the occasion of the above described transient operation period. The second approach is to suppress the shift of the local oscillation frequency f.sub.L on the occasion of the above described transient operation to such a range that does not cause malfunction, say approximately 4MHz, as seen from FIG. 4. However, the former approach requires a malfunction preventing circuit including a switching device and the like on/off controlled on the occasion of the above described transient operation, while the latter approach requires manual operation for adjusting for each receiver the difference of a variable range of the local oscillation frequency f.sub.L. Thus, these approaches ineviatably results in an increase of the manufacturing cost.